During　experiments　on　investigating　the　application　feasibility　of　variable-frequency　air　source　heat　pump　with　vapor　injection　(ASHPVI)　in　severe　cold　region,　it　was　found　its　frosting　performance　varied　significantly　from　that　in　former　investigations,　e.g.　in　a　severe　frosting　condition　(0　degrees　C,　80%),　the　experimental　unit　was　frost-free.　To　investigate　the　frosting　characteristics　of　variable-frequency　ASHPVI　for　space　heating　in　severe　cold　region,　a　test　rig　was　built　and　used　to　supply　heating　for　a　lab　in　two　heating　seasons.　A　new　frosting　map　was　developed　through　summarizing　its　running　performance　under　frosting　condition.　Results　showed　the　frosting　map　of　variable-frequency　ASHPVI　was　affected　by　heating　load.　Owing　to　the　consideration　of　heating　load,　the　critical　relative　humidity　decreased　from　80.60%　to　62.75%　when　outdoor　temperature　(OT)　dropped　from　1.0　degrees　C　to　-26.5　degrees　C,　opposite　to　the　change　trend　in　former　investigations.　Besides,　compared　with　values　obtained　in　cold　region,　the　upper　and　lower　limits　of　OT　of　frosting　region　in　severe　cold　region　were　both　lower,　and　its　critical　relative　humidity　was　higher.　Moreover,　the　highest　frosting　speed　appeared　at　OT　of　approximately　-10　degrees　C　in　severe　cold　region,　but　not　at　0　degrees　C　as　previously　thought.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.